|Garnet is the generic term for a family of minerals that share a similar formula and the same crystal structure. Most of the garnets are not pure, they have slight variations in formula and can be defined into smaller groups with two "end-members". The end members have specific formulas. but the actual stones have a mixture of the two end member formulas.
Pyrope garnet Mg3Al2(SiO4)3 and Almandite garnet Fe3Al2(SiO4)3 form one such series of garnets. Rhodolite garnet is a member of this chain roughly half way between the two end members. (Mg1.5, Fe1.5) Al2(SiO4)3 . All of these garnets are in the red/violet - red/brown color spectrum.
Spessartite garnet in the manganese analog Mn3Al2(SiO4)3 and is in the red-orange to red-brown color area.
Grossular garnets are the calcium rich varieties. Ca3Al2(SiO4)3 There are several garnets with this general formula, hessonite, hydrogrossular, and the best known tsavorite. Tsavorite is a bright green and very right garnet that can be confused for emerald in smaller stones.
Andradite garnets Ca3Fe2(SiO4)3 have a very high dispersion and refractive index and produce "diamond-like" stones. This series produces beautiful stones but rarely in the larger sizes. Melanite is a black variety and topazolite is a yellow variety. The Demantoid is the most prized and is another bright green variety.
Uvarovite is the chromium - calcium garnet Ca3Cr2(SiO4)3 and this garnet could be a real winner if it were ever found in larger sizes. Typically it found as 1-2 mm crystal coverings. It is very rarely fond large enough to facet. It is a beautiful green color but almost unknown in the jewelry trade. It can be found sometimes as a "drusy" coating and cut as a flat cabochon displaying the small crystals.